It looks like your problem is calcium fluoride scale caused by either too little dilution water and/or too much hardness in the water.
Sodium fluorosilicate is a crystalline powder with limited solubility in water. It takes 60 gallons of water to dissolve 1 lb. In areas with high hardness, a water softener is often needed on the make-up water feed to minimize the potential for calcium fluoride scale formation. To avoid scale formation in hard water, either a water softener is needed, or other fluoride chemical should be used to minimize scale formation. 10-State standards specifies water softening if hardness exceeds 75 mg/L as calcium carbonate.
Unlike sodium fluoride, which has a constant solubility, sodium fluorosilicate has a solubility that varies with temperature, from 0.44 g per 100 mL (which is 1/10th that of sodium fluoride) at 32°F to 2.45 g per 100 mL at 212°F. When the water temperature is 77°F, the solubility is approximately 0.762 g per 100 mL, or 1/5th the saturated solution strength of sodium fluoride at the same temperature. Consequently, it requires a lot more water to form a solution.
PVC pipe is relatively resistant to scaling. A velocity of 3-4 ft/sec will also not allow any solids to settle in the pipeline.
Carbon steel piping should be avoided as it will be attacked. PVC, CPVC or Polyethylene piping are commonly used for fluoride solutions, but polypropylene can also be used. PVC has been more commonly used, but its disadvantages are that it is a rigid connection with many joints for potential leaking where polyethylene is a flexible piping.
Polyethylene and Polyvinylchloride are the two pipe materials best suited for fluoride solution piping. They are both formed from ethylene gas, which as 2 carbon atoms, but are polymerized into long-strand carbon chains. PVC has one alternate hydrogen replaced by a chlorine atom. Their chemical resistance is approximately the equivalent, but their physical properties are quite different. PVC is historically the pipe material used for most facilities, and it is a rigid thermoset which needs to be “glued” using a chemical solvent. It will have many joints which are prone to leaking and as a rigid pipe, is subject to fracture. PE is a flexible pipe and as thermoplastic, can be joined by melting the edges to a continuous run without any leaks or potential leaking. However, this requires specialized equipment and experience by the fabricator.
